Carbon/metal composite

ABSTRACT

An improved carbon/metal composite is disclosed which comprises a carbon matrix and metal fibers distributed in the carbon matrix. The improvement is that the metal fibers are selected from (A) metal fibers in which the surfaces of at least a portion of the fibers are coated or alloyed with another material which has a tendency to form carbides which is equal to or lower than that of the metal constituting the metal fibers, (B) metal fibers comprising at least two different types of metal fibers which differ with respect to their dimensions and/or material, and (C) metal fibers distributed in the carbon matrix in such a manner that their content varies along the thickness of the composite, thereby imparting to the composite improved properties with respect to at least one of mechanical strength, impact resistance, wear resistance, and electric conductivity.

BACKGROUND OF THE INVENTION

This invention relates to a carbon/metal composite. More particularly,it relates to a carbon/metal composite having a high strength, good wearresistance, and good electrical properties and which is suitable for useas a sliding current collector such as a pantograph slider for electrictrains.

In the past, carbon materials and metallic materials such as cast alloysand sintered alloys have been employed for sliding current collectors.Metallic materials have the advantages of high strength and goodelectrical conductivity, but they have the drawbacks that they producemany arcs, their sliding properties are inadequate, and they produce agreat deal of wear in the trolley wires or third rails with which theyare in a sliding contact. Carbon materials have excellent lubricatingproperties and produce little wear of the wires or rails which theycontact, but they have a high electric resistance and easily heat upduring current collection.

The power consumption of railroad cars has been increasing due to highertrain speeds and the installation of air conditioning. In order to copewith the increased power consumption, in recent years, sliding currentcollectors have begun to be made from carbon/metal composites, whichcombine the excellent sliding properties of carbon with the electricalconductivity of metals.

Japanese Published Examined Patent Application No. 56-14732 (1981)discloses a carbon/metal composite which is formed by impregnation underpressure of a metal into the pores of a carbon material.

Japanese Published Unexamined Patent Application No. 60-238402 (1985)discloses a current collecting material made from a carbon materialcontaining metal powder which is obtained by adding a metal powder withgood conductivity to a carbon raw material.

Japanese Published Unexamined Patent Application No. 61-245957 (1986)discloses a manufacturing method for a current collecting material inwhich a mixture of a carbon aggregate with a binder which contains metalfibers and/or carbon fibers is molded and baked, after which the bakedbody is impregnated with a metal.

Japanese Published Unexamined Patent Application No. 62-72564 (1987)discloses a manufacturing method for a sliding current collectingmaterial in which metal fibers are blended into a carbon raw material,and the blend is molded at ambient temperature and then baked.

Japanese Published Unexamined Patent Application No. 62-197352 (1987)discloses a manufacturing method for a sliding current collecting carbonmaterial in which metal fibers are blended with a carbon raw material soas to be oriented unidirectionally, after which molding and baking areperformed.

Japanese Published Unexamined Patent Application No. 63-215731 (1988)discloses a manufacturing method for a carbon/metal composite frictionalmaterial for use in brakes in which pitch, metal fibers, and graphiteare mixed and then molded at a temperature of 450°-600° C. under amolding pressure of at least 40 kg/cm².

However, the great majority of carbon/metal composites produce a greatdeal of wear of sliding contact with trolley wires, normally made ofcopper, when the surfaces of the wire are in a roughened condition. Sucha roughened surface of trolley wires will be usually observed whilepantograph sliders formed of a sintered metal which are at presentprevailing is being replaced by those of a carbon/metal composite in thefuture, i.e., during a period when sliders of these two types are usedconcurrently.

Furthermore, most of the conventional carbon/metal composites areinferior to conventional metal sliders with respect to bending strengthand impact strength. It is easy for sliders made from carbon/metalcomposites to be chipped or broken by collision with a hanger ear of atrolley wire which has been detached due to vibration or shock caused byrunning trains or any other accidental cause, so they are less safe andless reliable than conventional metal sliders.

Furthermore, carbon/metal composites have a higher electric resistancethan conventional metal sliders. A high electric resistance leads to anincrease in the temperature of trolley wires due to Joule heating. Thetemperature increase is particularly significant when a train is stoppedbut the air conditioning and interior lighting of the train are stilloperating. A high temperature may cause the breakage of the trolleywires under high tension, which is extremely dangerous.

It is possible to improve the strength and electrical properties of acarbon/metal composite by increasing the metal content of the composite.However, as the metal content is increased, more sparks are generatedbetween the slider and the trolley wires, and the wear of the trolleywires and the slider is increased, which is undesirable.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide acarbon/metal composite having a good mechanical strength such as bendingstrength and impact strength, as well as good wear resistance.

It is another object of the present invention to provide a carbon/metalcomposite suitable for use as pantograph sliders which has a goodmechanical strength, good wear resistance, and a low electric resistanceand which does not generate many sparks when in moving contact with atrolley wire.

It is still another object of the present invention to provide acarbon/metal composite which is minimized in wear loss when it is insliding contact with a trolley wire having a roughened surface.

It is a further object of the present invention to provide a method forthe manufacture of a carbon/metal composite having the above-describedproperties.

Other objects as well as the nature and the advantages of the presentinvention will be apparent from the following description.

In one aspect of the invention, there is provided an improvedcarbon/metal composite comprising a carbon matrix and metal fibersdistributed in the carbon matrix. The improvement is that the metalfibers are selected from (A) metal fibers in which the surfaces of atleast a portion of the fibers are coated or alloyed with anothermaterial which has a tendency to form carbides which is equal to orlower than that of the metal constituting the metal fibers, (B) metalfibers comprising at least two different types of metal fibers whichdiffer with respect to their dimensions and/or material, and (C) metalfibers distributed in the carbon matrix in such a manner that theircontent varies along the thickness of the composite, thereby impartingto the composite improved properties with respect to at least one ofmechanical strength, impact resistance, wear resistance, and electricconductivity.

In another aspect, there is provided a method for manufacturing acarbon/metal composite which comprises: forming a molding mixturecomprising a carbon raw material and at least one component selectedfrom (a) metal fibers at least a portion of which are coated withanother material having a tendency to form carbides which is equal to orlower than that of the metal constituting the metal fibers, (b) metalfibers at least a portion of which have surfaces alloyed with anothermetal having a tendency to form carbides which is equal to or lower thanthat of the metal constituting the metal fibers, and (c) a mixture ofmetal fibers and a metal powder, said metal powder having a tendency toform carbides which is equal to or lower than that of the metalconstituting the metal fibers; molding the molding mixture to form amolding; and baking the molding to carbonize the carbon raw material,thereby forming a carbon/metal composite.

If desired, the metal fibers can be oriented in a substantiallyunidirectional manner. Unidirectional orientation or alignment of themetal fibers further lowers the electric resistance and increases theimpact strength of the composite.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of a molding apparatus which can be employedfor manufacturing a carbon/metal composite according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A carbon/metal composite according to this invention has metal fibersdistributed in a carbon matrix. It is manufactured by preparing amolding mixture which comprises a carbon raw material and metal fibers,molding the mixture, and then baking the resulting molding to carbonizethe carbon raw material.

Various carbon raw materials can be employed to form a carbon/metalcomposite according to this invention. Some examples are (1) aself-sintering mesophase powder, (2) a binary raw material which is amixture of a carbonaceous aggregate such as coke powder with pitch whichfunctions as a binder, and (3) a carbonizable thermosetting resin suchas a phenolic resin.

From the standpoint of wear resistance, the carbonaceous aggregate inthe binary raw materials is preferably a hard carbonaceous carbonmaterial such as coke powder which can be obtained by carbonizing pitchor coal at around 1000° C. or an isotropic carbon powder which can beobtained by carbonizing a phenolic resin.

From the standpoints of strength and wear resistance, the carbonaceousaggregate is ground to a particle size of preferably at most b 50 μm andmore preferably at most 20 μm.

The pitch which is employed as binder can be coal tar pitch, or pitchwith a high softening point which is obtained by heating coal tar pitch.

The pitch preferably has fluidity when heated. Furthermore, in order toincrease the strength and wear resistance of the resulting composite,the volatile content of the pitch is preferably as low as possible.

When the composite is to be used for a pantograph slider, a binarycarbon material comprising the above-described hard carbonaceousaggregate and pitch as a binder as primary components is desirable fromthe standpoints of cost and performance.

The metal fibers can be formed by a variety of methods, such as by thethin sheet slicing method, the chatter vibration cutting method, thewire cutting method, or by drawing. The shape of the individual metalfibers is not critical. For example, they can be rod-shaped,needle-shaped, wedge-shaped, wave-shaped, net-shaped, or a mixture offibers having the above shapes.

There is not particular restriction of the material used for the metalfibers. The fibers can be steel fibers manufactured from common steel,high tensile steel, stainless steel, and the like. Metals other thansteel can also be used, such as copper. Steel fibers made fromlow-carbon steel exhibit the best properties. The presence of manganeseor chromium in steel fibers promotes cementation of the steel, whichdecreases the strength of the resulting composite. Therefore, thecontent of these elements is preferably as low as possible.

The dimensions of the metal fibers are not critical, but from thestandpoint of obtaining a strong molding, the diameter of the fibers ispreferably at most 0.5 mm and more preferably at most 0.3 mm. The fiberlength is preferably at least 1 mm and more preferably at least 3 mm.Strictly the diameter of a fiber should be expressed as the equivalentdiameter that is the diameter of a circle having the samecross-sectional area as the fiber.

As will be described in greater detail further on, when using two ormore types of metal fibers having different dimensions, it is possiblefor some of the fibers to have a diameter of up to around 1 mm.Generally, the aspect ratio of the metal fibers (the ratio of fiberlength to fiber diameter) is preferably at least 10. Although extremelyelongate fibers having an aspect ratio exceeding 100 may be used, theytend to become entangled when blending with a carbon raw material.Therefore, if they are used, it is preferred that the amount of thesefibers be not so large.

There is no particular restriction on the amount of the metal fibers. Inorder to improve the wear resistance, mechanical strength, and electricconductivity of the resulting composite, it is preferable that the metalfibers be present in an amount of at least 10 volume % in the composite.A larger amount of metal fibers on the order of 50 to 60 volume % may beemployed, although the presence of metal fibers in an amount exceedingabout 50 volume % tends to produce many sparks when the composite isused as a pantograph slider, thereby increasing the wear rate of thecomposite. Preferably the amount of the metal fibers is in the range offrom 10 to 40 volume %, and more preferably in the range of from 15 to35 volume % of the composite.

The present inventors discovered that the reason why conventionalcarbon/metal composite sliders having a low bending strength is thatduring the baking stage of manufacture of the composite, the metalfibers are carburized by the surrounding pitch and carbon powder, andmetal carbides are formed in the fibers. For example, steel fibers areconverted into a composition containing a large amount of cementite (Fe₃C). The carburization to form a metal carbide may be hereunder referredto as cementation.

Carburization or cementation of metal fibers such as steel fibers doesnot occur to a great extent at a baking temperature of less than 900° C.However, the baking stage in which the carbon raw material is carbonizedto form a carbon matrix is normally performed in the vicinity of 1000°C. so that the resulting carbon matrix can fully exhibit their strength,and at this temperature, cementation of the fibers is greatlyaccelerated. The cementite which is formed by cementation of steelfibers is hard and brittle. A composite containing fibers of cementitehas good wear resistance, but its toughness is low, and the bendingstrength is significantly decreased. Therefore, it is desirable tosuppress cementation of the metal fibers during baking.

The present inventors discovered that the following measures areeffective for suppressing cementation of metal fibers such as steelfibers.

(1) Prior to molding, if the surfaces of steel fibers are coated withanother material such as copper, nickel, cobalt, aluminum, or siliconwhich has a tendency to form carbides which is equal to or lower thaniron, the cementation of the steel fibers can be suppressedsignificantly, and the strength of the resulting composite can begreatly increased.

(2) Even if the steel fibers are not coated, if a metal powder mainlycomprising a metal with such a low tendency to form carbides is added tothe steel fibers, the cementation of the steel fibers can be suppressed.

(3) The cementation of steel fibers can also be suppressed by alloyingat least the surfaces of the steel fibers with another metal having alow tendency to form carbides.

(4) If the surfaces of only a portion of the steel fibers are coated oralloyed with such a material having a low tendency to form carbides, thecementation of steel fibers can be substantially suppressed.

According to one embodiment of the present invention, the surfaces of atleast a portion of the metal fibers which are present in the compositeare coated or alloyed with another material which has a tendency to formcarbides which is equal to or lower than the metal fibers, therebyimparting improved mechanical strength to the carbon/metal composite.

The coating of metal fibers can be applied by any conventional methodsuch as vapor deposition, but typically, it is applied bynon-electrolytic plating. Cementation of the metal fibers can beeffectively suppressed even if the coating has a thickness of only 0.1μm. There is no exact upper limit on the thickness of the coating, butfrom the standpoint of economy, a thickness of at most 10 μm isgenerally suitable. The thickness is preferably 0.1-5 μm and morepreferably 0.2-2 μm.

Any material which has a tendency to form carbides which is equal to orlower than the metal constituting the metal fibers can be used as acoating material. When the resulting composite is to be used as apantograph slider, both excellent resistance to cementation and a lowelectric resistance are necessary, so the coating material is preferablya metal. When the metal fibers are steel fibers, preferred coatingmaterials are such metals as copper, nickel, cobalt, aluminum, andsilicon. If the coating material has a high electric resistance, themetal fibers will be prevented from improving the conductivity of thecomposite, so the electric resistance of the composite becomes extremelyhigh, and the composite will be unsuitable for use as a pantographslider.

However, when the composite is to be used as a brake material or othertype of sliding member which does not carry current, the conductivity isnot important, and ceramics such as alumina, silicon carbide, and silicawhich has a high electric resistance can be employed as a coatingmaterial. A coating of a ceramic can be applied by any suitabletechnique such as plasma spray coating.

When the surfaces of metal fibers are coated with a material having alow tendency to form carbides, the cementation of the metal fibers iseffectively suppressed during the baking stage. As a result, thestrength, and particularly the bending strength, of the resultingcomposite is greatly increased.

For this purpose, of course, it is possible to coat the surfaces of allthe metal fibers with another material as above. However, while surfacecoating increases the bending strength of a composite, it decreases theamount of cementite formed in the fibers, which due to its hardnessincreases the wear resistance of the composite. Therefore, coating allthe metal fibers tends to decrease the wear resistance of the resultingcomposite.

This decrease in the wear resistance can be prevented if a portion ofthe metal fibers have uncoated surfaces. The uncoated metal fibers reactwith carbon in the surrounding carbon raw material during baking andform cementite which has excellent wear resistance. As a result, acomposite can be obtained which has good bending strength withoutsacrificing wear resistance.

When coated and uncoated metal fibers are used together, there is notparticular limit on the ratio of coated to uncoated fibers. However,cementite can greatly improve wear resistance even when present in onlya small quantity. Therefore, in order to maintain a good bendingstrength, it is desirable to use a large amount of coated metal fibers.The coated metal fibers preferably constitute at least 50 weight % andmore preferably at least 65 weight % of the total amount of metalfibers.

The cementation of the metal fibers during the baking stage can also beeffectively suppressed by adding to the carbon raw materials a powder ofa metal which has a tendency to form carbides which is equal to or lowerthan the metal fibers. As a result, the strength of the composite can beincreased.

Useful metal powder can be formed from any metal having such a lowtendency to form carbides. When the metal fibers are steel fibers,suitable metal powders include those formed from copper, nickel,aluminum, cobalt, and silicon.

The element distribution of a carbon/metal composite which was preparedfrom a molding mixture containing a metal powder was investigated usingan electron probe micro analyzer (EPMA). Elements of the metal powderwere found inside the metal fibers. When X-ray diffraction analysis wasperformed, almost no cementite was found. These results show that whenthe molding is baked to carbonize the carbon raw material, the metalpowder added is spread into the metal fibers to form an alloy at leastnear the surfaces of the fibers, and the cementation of the metal fibersis thereby prevented.

When the metal powder is formed from a metal such as manganese ofchromium which has a great tendency to form carbides, the strength ofthe resulting carbon/metal composite is decreased as compared to acomposite to which no metal powder is added. The reason for thisdecrease in strength is that cementation is accelerated, resulting in adecrease in the strength of the metal fibers themselves. Furthermore,voids are formed between the metal fibers and the carbon matrix, causinga decrease in the bond strength between the fibers and the matrix.

The metal powder may contain one or more elements which have a tendencyto form carbides equal to or lower than the metal fibers. When itcontains more than one of such elements, the powder may be made of analloy of these elements, or it may be a mixture of the respective metalpowders of these elements.

The purity of the metal powder is not critical unless it contains alarge amount of undesirable elements such as manganese or chromium.Thus, any metal powder may be used which contains a major amount of ametal having a low tendency to form carbides. For example, an alloy suchas ferronickel can be employed.

The average particle size of the metal powder is preferably at most 100μm and more preferably from 0.5 to 50 μm. If the average particle sizeis greater than 100 μm, cracks can easily form in the periphery of themetal particles. Furthermore, the number of points of contact betweenthe metal particles and the metal fibers is decreased, so there isreduced formation of alloys in the fibers which serve to suppresscementation.

When metal powders having a low tendency to form carbides are employed,the metal fibers may be uncoated. Due to the effect of the addition ofmetal powder, a composite having a satisfactory strength can be obtainedeven if the metal fibers are uncoated. However, it is possible for allor part of the metal fibers to be coated so as to obtain a furtherincrease in the bending strength. When coated metal fibers and metalpowders are both employed, the metal constituting the coating and themetal constituting the metal powder can be the same or different fromone another, as long as both metals have a tendency to form carbideswhich is equal to or lower than the metal constituting the metal fibers.

The metal powder may be simply added to the carbon raw material togetherwith the metal fibers and then mixed by usual methods. However, thestrength of the resulting composite can be increased if the metal fibers(either coated or uncoated) are first mixed alone with the metal powderin order to adhere the metal powder to the metal fibers, and then themetal fibers and metal powder are mixed with the carbon raw material.When mixing is performed in this manner, the bond strength between themetal powder and the metal fibers can be increased by the addition of asmall quantity of a resinous or oily binder or a surfactant.

There is no particular restriction on the amount of metal powder whichcan be added. However, when the metal fibers are totally uncoated, theamount of metal powder is preferably 0.5-20 volume % and more preferably1-10 volume % of the molding mixture. When the metal fibers are coated,the amount of metal powder can be decreased.

As mentioned above, the addition of a metal powder can suppress thecementation of the metal fibers through the formation of an alloy in thefibers during the baking stage of manufacture. Therefore, cementation ofthe metal fibers and an accompanying decrease in strength of theresulting composite can be prevented by using metal fibers the surfaceof which have been alloyed, prior to use, with a metal having a tendencyto form carbides which is equal to or lower than the metal fibers.

One method of alloying prior to use is to coat the surface of the metalfibers, such as steel fibers, with one or more alloying metal such ascopper, nickel, cobalt, aluminium, or silicon which have a low tendencyto form carbides and then to heat the coated steel fibers at a hightemperature to diffuse the alloying metal into the metal fibers to forman alloy at least near the surface of the metal fibers. Alternatively,uncoated metal fibers can be mixed with a metal powder containingpredominantly one or more of the above alloying metals and then heatedto a high temperature to perform alloying.

In either case, the heat treatment in order to perform alloying shouldbe carried out at a temperature lower than the melting point of themetal fibers, but it should be carried out long enough and at highenough temperature for a substantial amount of the alloy element todiffuse into the metal fibers. When the metal fibers are steel fibers,heat treatment is preferably carried out at a temperature ofapproximately 600°-1100° C. for approximately 30 minutes-3 hours.

Even if heat treatment is not carried out in order to perform alloyingprior to mixing of the fibers with the carbon raw material, during thebaking stage in which the molding is baked in order to carbonize thecarbon raw material, the metal fibers are exposed to a temperatureequivalent to that used in the heat treatment for a considerable lengthof time. Therefore, if all or a portion of the metal fibers are coatedwith a metal having a low tendency to form carbides, and/or if a metalpowder having a low tendency to form carbides is added to the metalfibers, then in the baking stage of manufacture, the coating or addedmetal powder will be alloyed with the metal fibers to a certain extent.As a result, the formation of cementite will be suppressed, and thestrength will be improved.

According to one embodiment of this invention, metal fibers are blendedwith a carbonaceous raw material so as to orient the fibers insubstantially unidirectional alignment. The metal fibers may beuncoated, or all or part of the fibers may be coated. If the metalfibers are aligned in this manner, the continuity of the metal fibers inthe resulting carbon/metal composite is increased, so the electricresistance is enormously decreased, and the electrical properties of thecomposite are improved. Furthermore, the metal fibers effectively act asreinforcing members, so the impact resistance of the composite isgreatly increased. Furthermore, in a bending test, even after the yieldpoint is exceeded, the composite bends instead of failing, so itexhibits an extremely high bending strength.

The following methods can be used to orient the metal fibers so as to besubstantially unidirectional in the carbon raw material.

(1) When the metal fibers are made of a ferromagnetic substance such assteel fibers, after a molding mixture of the metal fibers and the carbonraw material is placed into a mold, a magnetic field of at least severalhundreds of Gauss is applied to the molding mixture. The magnetic fieldaligns the fibers in the direction of the field. After orientation hasbeen performed, molding is carried out.

(2) When the metal fibers are made of copper or similar material whichis not ferromagnetic, the metal fibers are formed into a cloth in whichthe warp length is far greater than the woof length so as to provide asubstantial unidirectional orientation. The cloth is then laminated withthe carbon raw material, placed into a mold, and molded.

(3) When the metal fibers are short with a length of at most 5 mm, amixture of the carbon raw material and the short fibers is passedthrough slits having a width which is smaller than the length of thefibers. The mixture is then placed into a mold and molded.

(4) Molding is carried out by extrusion.

In another embodiment of the present invention, a mixture of at leasttwo different types of metal fibers is used which differ from oneanother in one or more characteristics selected from their dimensions(length and diameter) and material. The fibers can be uncoated, or allor part of the fibers may be coated or alloyed with another materialwhich has a tendency to form carbides which is equal to or lower thanthe metal constituting the fibers.

Many of the carbon/metal composites which have been proposed as amaterial for pantograph sliders in the prior art have a much higherelectric resistance and a much lower impact strength than conventionalsliders made from sintered metal. The drawbacks of conventionalcarbon/metal composites are due to the fact that the carbon matrix has ahigh electric resistance and a low impact strength. These problems canbe overcome by increasing the content of metal fibers in a composite.However, as the metal content of a slider increases, the generation ofsparks between the slider and trolley wires increases, and the wear ofthe trolley wires and the slider increases, which is undesirable.

As a result of investigations aimed at obtaining a carbon/metalcomposite having good wear resistance and good impact resistance withoutincreasing the content of metal fibers, the following discoveries weremade.

Metal fibers have a high aspect ratio, so they act as reinforcingmembers and increase the static strength and impact strength of acomposite. For fibers having the same aspect ratio, the smaller thefiber diameter the higher is the static strength. In contrast, theimpact strength increases with the length of the fibers, and if theaspect ratio is at least 10, the larger the fiber diameter the higher isthe impact strength.

Furthermore, for fibers of the same material, it is well known that thehigher the aspect ratio, the higher are the static strength and theimpact strength of the composite containing the fibers. However, if theaspect ratio is too high, when the fibers are mixed with a carbon rawmaterial, it becomes easy for the fibers to become entangled and form anagglomeration referred to as a fiber ball. In this case, it is necessaryto reduce the fiber content, which is disadvantageous.

The present inventors found that the use of two or more different typesof fibers is effective for increasing both the static and impactstrength of a composite.

For example, if metal fibers having a small diameter are mixed with asmall amount of long metal fibers having a large diameter, the staticstrength of the resulting composite is somewhat decreased, but theimpact strength is enormously increased. A similar effect can beobtained by mixing two or more types of metal fibers having the samediameter but different lengths.

When combining metal fibers of different lengths, the ratio of thelength of the longer fibers to the length of the shorter fibers ispreferably at least 2. The amount of long fibers is preferably 10-40parts by volume with respect to 100 parts by volume of short fibers.

It is also possible to mix metal fibers which are made of differentmaterials. For example, if ferrous metal fibers which produce good wearresistance and mechanical strength are combined with copper fibers whichhave a low electric resistance, it is possible to obtain a compositehaving good wear resistance and mechanical strength as well as a lowelectric resistance. When using metal fibers made of two or moredifferent materials, the materials can be chosen in accordance with theproperties which are desired of the resulting composite.

In accordance with the present invention, it is also possible to varyone or more of the content, the material, or the dimensions (diameter orlength) of the metal fibers along the thickness of the composite. Therequirements of a composite with respect to wear resistance, impactresistance, and electric resistance may not be the same for both side ofthe composite. For example, the metal fibers may be distributed so as tobe more concentrated near one surface of the composite which is desiredto contain the fibers. When two or more different type of metal fibersare used, they may be distributed unevenly along the thickness such thateach type of fibers are present so as to be more concentrated near onesurface. Therefore, fibers which tend to improve a certain property canbe provided in greater quantities near the surface of the compositewhich requires the property than near the other surface for which theproperty is less important. As the result, the overall properties of thecomposite can be improved over the composite in which the metal fibersare distributed uniformly throughout.

In the case of a pantograph slider, the electric resistance is extremelyimportant. However, if the content of metal fibers is increased toobtain a low electric resistance, the wear resistance decreases, so itis difficult to obtain a satisfactory electric resistance and asatisfactory degree of wear resistance at the same time.

However, it is possible to obtain a composite having both a low electricresistance and good wear resistance by using two type of metal fibers ofdifferent materials which are present with distributions varying alongthe thickness of the composite rather than a uniform distributionthroughout. The fibers which have a low electric resistance but whichtend to lower the wear resistance are concentrated near one surface ofthe composite, while the other metal fibers are concentrated near theother surface.

In a pantograph slider, for example, the upper side which is in slidingcontact with trolley wires requires good wear resistance and a lowtendency to generate sparks. Therefore, ferrous metal fibers such assteel fibers and more preferably low-carbon steel fibers which providegood wear resistance can be provided in greater quantities in the upperportion. On the other hand, the lower portion of the slider which doesnot contact trolley wires can contain metal fibers such as copper fiberswhich have a low electric resistance. The resulting pantograph sliderwill have good wear resistance, a low electric resistance, and a lowtendency to generate sparks.

By varying the distribution (content) of metal fibers along thethickness of a molding so that there are portions with many fibers andportions with few fibers, the impact resistance, wear resistance, andtendency to generate sparks of a composite can be simultaneouslyimproved without an increase in the total amount of metal fibers whichaccompanies a deterioration in the wear resistance.

Again taking a pantograph slider as an example, the amount of metalfibers in the upper portion which contacts trolley wires can be reducedin order to improve wear resistance and decrease the generation ofsparks, while in the lower portion which does not contact trolley wires,the amount of metal fibers can be increased in order to increase impactresistance and decrease the electric resistance. This type of slider hassuperior wear resistance and impact resistance and generates fewersparks than a slider having a uniform distribution of metal fibersthroughout.

When the content of metal fibers is varied in this manner, the contentis preferably lower in the upper portion of the slider corresponding tothe thickness which will be worn by sliding contact with the trolleywires than in the other portion. The content of metal fibers in theupper portion is preferably 10-40 volume % and more preferably 10-35volume %, while the content of metal fibers in the other portion ispreferably 40-65 volume % and more preferably 10-50 volume %.

It is also possible to employ two or more different types of metalfibers which differ from one another with respect to their dimensions(length and/or diameter) and to distribute the different types unevenlyalong the thickness of a composite. Such a composite has superior wearresistance, impact strength, and bending strength compared to acomposite having the same content of metal fibers which are distributeduniformly throughout.

As mentioned above, when the material and content of metal fibers areconstant, the longer are the metal fibers, the higher is the impactstrength of the resulting composite. Furthermore, when the aspect ratioof the fibers is at least 10, the impact strength also increases as thefiber diameter increases. However, metal fibers which are long or have alarge diameter decrease the wear resistance of the resulting composite.

For example, in the case of a pantograph slider made from a carbon/metalcomposite, it is effective to dispose short fibers with a small diameterin the upper portion of the slider, which contacts trolley wires, inorder to increase its wear resistance, and it is effective to disposelong fibers with a large diameter in the lower portion which does notcontact trolley wires in order to give it a high impact strength. Inthis case, the wear resistance and the impact strength of the slider aresuperior to those of a pantograph slider which is made from a compositecontaining metal fibers of a single size. Furthermore, the slider hasless tendency to generate sparks.

Metal fibers which are effective in improving wear resistance preferablyhave a fiber diameter (equivalent diameter) of at most 0.3 mm andpreferably at most 0.1 mm, and a length of at most 10 mm. Metal fiberswhich are effective in improving impact resistance have an equivalentdiameter and a length which are both at least two times those of metalfibers which are effective in improving wear resistance. When the aspectratio of metal fibers is greater than 10, the larger the fiber diameterthe higher is the impact strength of the composite. However, if thefiber diameter (equivalent diameter) exceeds 1 mm, the bonding betweenthe metal fibers and the carbon worsens, and there is a tendency for thebend strength to decrease. Therefore, the equivalent diameter of thickermetal fibers which are added for the purpose of increasing impactresistance preferably does not exceed 1 mm.

In a composite which is formed using a mixture of two or more differenttypes of metal fibers which differ with respect to their dimensionsand/or material, or in a composite in which at least one of the content,material, and dimensions is varied along the thickness of a composite,the metal fibers may be uncoated, or they may be coated or alloyed withanother material having a tendency to form carbides which is equal to orlower than the metal fibers, just as in a composite which employs only asingle type of metal fibers. Also, it is possible to add to the metalfibers a metal powder having such a low tendency to form carbides. Asdiscussed previously, the use of coated or alloyed metal fibers or theaddition of a metal powder increases the bending strength of thecomposite.

If desired, all or part of the different types of metal fibers can beoriented in substantially unidirectional alignment. For example, in aslider having ferrous metal fibers in the upper portion which contactstrolley wires and having copper fibers in the lower portion, theelectrical properties of the composite can be improved by substantialunidirectional orientation of the copper fibers.

The carbon/metal composite of the present invention is prepared bymixing a carbon raw material with one or more types of metal fibers (allor a part of which may be coated or alloyed with another material asmentioned above) and an optional one or more metal powders, if any isused, to form a molding mixture, then molding the mixture, and bakingthe resulting molding to carbonize the carbon raw material.

In the mixing stage or molding stage of manufacture, the fibers can beoriented substantially unidirectionally in the manner described above.The content, material, and/or dimensions of the metal fibers can bevaried along the thickness of the molding by preparing a plurality ofmixtures of metal fibers and carbon raw materials, each mixturediffering with respect to the content, material, or dimensions of themetal fibers present therein. The different mixtures can then be placedin a prescribed order into a mold and molded.

Molding can be performed by various conventional methods, such asextrusion, cold isostatic pressing (CIP), cold molding, or hot pressing.Of these methods, hot pressing using a binary carbon raw material (amixture of coke powder and pitch) gives a composite with the higheststrength and wear resistance.

During hot pressing, it is desirable to heat the molding mixture underpressure to a temperature at which the pitch will harden, i.e., at least480° C. in order to obtain good strength and wear resistance in theresulting composite. Therefore, the heating temperature is preferably atleast 480° C. and more preferably at least 500° C. The possible highesttemperature during hot pressing is approximately 600° C. If thistemperature is exceeded, then cracks tend to form in the molding. Duringat least a portion of the time when the mixture is being heated fromroom temperature to the hot pressing maximum temperature, a moldingpressure of preferably at least 40 kg/cm² and more preferably at least80 kg/cm² is applied to the mixture. If the molding pressure is lessthan 40 kg/cm², the bond strength between the binder and the metalfibers decreases, and there is a tendency of the wear resistance todecrease.

FIG. 1 is a schematic view of an example of a molding apparatus for hotpressing. In the FIGURE, 1 is a movable upper press head, 2 is astationary lower press head, 3 is an upper mold, 4 is a lower mold, 5 isa metal frame, 6 is a molding mixture, 7 is a heating plate containing asheath heater 7-1, and 8 is a thermal insulating member. After themolding mixture 6 is placed between the upper mold 3 and the lower mold4, the heating plate 7 is heated by passing current through the sheathheater 7-1, and the molding mixture is pressed by the upper press head.The upper and lower molds can be preheated if desired.

The resulting molding is backed by heating it in a non-oxidizingatmosphere at a temperature below the melting point of the metal fibers.The baking carbonizes the carbon raw material, and a carbon/metalcomposite in which metal fibers are distributed in a carbon matrix isobtained. The baking temperature is preferably at least 900° C. so thatthe carbon can adequately exhibit its strength. However, if the bakingtemperature exceeds 1100° C., when the metal fibers are coated withanother metal, the coating layer may melt and decrease in effectiveness.Furthermore, the metal fibers themselves may undergo a transformationwhich causes a decrease in the strength of the composite. Therefore, thebaking temperature is preferably 900°-1100° C. and more preferably950°-1050° C.

A carbon/metal composite according to the present invention isparticularly suitable for use as a pantograph slider on account of itsstrength, wear resistance, and electrical properties. However, it canalso be used for other types of sliding current collectors such ascurrent collecting brushes, or for other types of sliding parts such asfriction members for friction brakes, bearings, and sealing members.

The present invention will now be described in further detail be meansof the following examples, which are given merely for the purpose ofillustration.

In the examples, the softening point of the pitch which was used as abinder was measured using a flow tester manufactured by ShimadzuSeisakusho.

When the metal fibers were plated, the average plating thickness on thefibers was calculated on the basis of the nominal dimensions of thefibers, the coated weight of the plating, and the true specificgravities of the metal constituting the metal fibers and the platingmetal.

The bending strength of the carbon/metal composites obtained in theexamples are measured using test pieces of 10 mm×60 mm×10 mm thick insize cut from the composites. When the thickness of the molding isroughly 10 mm, the thickness of the test pieces was the same as thebaked composite. The 60 mm side of the test piece was cut parallel tothe lengthwise direction of the composite. The benign strength was thenmeasured by a three-point bending test using a 40 mm span. The loadduring the bending test was applied in the same direction as the loadwas applied by the molding press during molding.

The impact strength was measured by a Charpy impact test using testpieces which measured 10 mm×60 mm×10 mm thick and which were cut in themanner described above. The direction of impact during the test wasperpendicular to the direction of load applied by the molding pressduring molding.

Wear resistance of the composite was measured using a pin-on-disk weartester. Test pieces measuring 8 mm long×8 mm wide were used. The 8 mm×8mm surface was used for testing and was contacted with a copper platehaving a diameter of 300 mm (diameter of contacting portion=132 mm,roughness of contacting surface=90 μm). The copper plate was forced tocontact the test piece under a prescribed load and rotated for aprescribed length of time, after which the reduction in the thickness ofthe worn surface of the test piece was measured. The wear resistance wasexpressed as the reduction in thickness or as the amount of wear (wornvolume) per 100 km of sliding.

The electric resistance was measured on test pieces measuring 10 mmwide×60 mm long×10 mm thick. Measurement was performed using thefour-terminal method in which current was made to flow in the lengthwisedirection of the test pieces.

EXAMPLE 1

In this example, a composite was manufactured using metal fibers coatedwith copper plating.

Regular grade petroleum coke was carbonized at 1000° C., after which itwas placed into an oscillating mill containing stainless steel ballswith a diameter of 10 mm. Grinding was performed for 4 hours to obtaincoke powder with an average particle diameter of 11.5 μm, which was usedas an aggregate of a binary carbon raw material for the preparation of amolding mixture.

Pitch, which was used as a binder, was obtained by heating coal tarunder a reduced pressure of 60 mmHg at 440° C. for 1 hour. The resultingcoal tar pitch had a softening point of 250° C. and was used aftergrinding to a size of 60 mesh or smaller.

The metal fibers were low carbon steel fibers (SPCC-1B) measuring 0.1mm×0.1 mm×3 mm long. The steel fibers were immersed at room temperaturein 80 times their weight of a plating solution having the compositionshown in Table 1 for the time shown in Table 2 to form copper plating.The fibers were then removed from the plating solution, thoroughlywashed with water, then rinsed with acetone, and dried in nitrogen at100° C. to obtain copper-plated steel fibers. The average platingthickness is also shown in Table 2.

22 parts by weight of coke powder, 10 parts by weight of pitch as abinder, and 68 parts by weight of the steel fibers were mixed to preparea molding mixture. The steel fibers constituted roughly 30 volume % ofmolding.

This molding mixture was molded using the hot press shown in FIG. 1. 350g of the molding mixture were placed into a steel mold with an innerdiameter of 100 mm. The molding mixture as heated without theapplication of pressure at a rate of 10° C./minute to 350° C. From 350°C., it was subjected to a pressure of 220 kg/cm² while being heated at arate of 5° C./minute to 540° C. A pressure of 220 kg/cm² was thenmaintained at 540° C. for 1.5 hours. The resulting molding was removedfrom the mold after cooling.

The molding was then baked by placing it in coke powder and heating in anitrogen gas atmosphere up to 480° C. at a rate of 100° C./hour. It wasthen maintained at 480° C. for 2 hours, after which it was heated at arate of 15° C./hour to 1000° C. This temperature was maintained for 3hours to carbonize the carbon raw material. After cooling, acarbon/steel composite was obtained.

The bending strength of the resulting composites is shown in Table 2.For comparison, the bending strength of a carbon/steel compositecontaining steel fibers which were not coated with copper plating isalso shown in Table 2.

From Table 2, it can be seen that a composite containing steel fiberswhich are coated with a copper plating has a far higher bending strengththan a composite containing uncoated steel fibers.

                  TABLE 1                                                         ______________________________________                                        Distilled water         1000   ml                                             Copper nitrate          15     g                                              Sodium hydrogen carbonate                                                                             10     g                                              Rochelle salt           30     g                                              Sodium hydroxide        20     g                                              Folmalin (37%)          100    ml                                             ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                 Duration of  Average plating                                                                           Bending                                              immersion    thickness   strength                                    Run No.  (hr)         (μm)     (kg/cm.sup.2)                               ______________________________________                                        Comparative                                                                            --           --          1070                                        This   1     0.5          0.09      1420                                      Invention                                                                            2     1.0          0.24      1590                                             3     2.0          0.40      1720                                             4     4.0          1.10      1670                                      ______________________________________                                    

EXAMPLE 2

In this example, a composite was manufactured using nickel-plated steelfibers.

The method of Example 1 repeated with exception that the steel fiberswere plated with nickel plating instead of copper plating. Steel fiberslike those used in Example 1 were plated by a non-electrolytic platingmethod using the plating solution shown in Table 3. Plating wasperformed by immersing the steel fibers in 100 times their weight of theplating solution for 20 minutes. During plating, the pH of the platingsolution was adjusted to 9 by the addition of an aqueous ammonia, andthe temperature was maintained at 85° C. After the completion ofplating, the steel fibers were washed and dried in the same manner as inExample 1 to obtain nickel-plated steel fibers. The average thickness ofthe plating layer was 1.2 μm.

A carbon/metal composite containing 30 volume % of the nickel-platedsteel fibers was then manufactured. It had a bending strength of 1800kg/cm².

                  TABLE 3                                                         ______________________________________                                        Distilled water        1000   ml                                              Nickel chloride        30     g                                               Sodium citrate         100    g                                               Ammonium chloride      50     g                                               Sodium hypophosphite   10     g                                               ______________________________________                                    

EXAMPLE 3

In this example, a composite was manufactured using both coated fibersand uncoated fibers.

Steel fibers like those used in Example 1 were immersed for 2 hours at20° C. in 10 times their weight of a plating solution having thecomposition shown in Table 1. After immersion, they were washed anddried in the same way as in Example 1 to obtain copper-plated steelfibers. The average plating thickness was calculated to be approximately0.5 μm.

The resulting copper-plated steel fibers, uncoated steel fibers, or amixture of copper-plated and uncoated steel fibers were mixed with thesame type of coke powder and pitch as were used in Example 1 in theproportions shown in Table 4 to form a molding mixture. In each case,the total amount of fibers was roughly 30 volume % of the molding. 450 gof each molding mixture were placed into a steel mold with innerdimensions of 100 mm×100 mm and molded by hot pressing.

During molding, the temperature of the molding mixture was increased ata rate of 3° C./minute. From room temperature to 300° C., a pressure of1 kg/cm² was applied, and from 300° C. to 550° C., a pressure of 200kg/cm² was applied. After maintaining the temperature at 550° C. under apressure of 200 kg/cm² for 1 hour, the resulting molding was cooled andremoved from the mold.

The molding was baked for carbonization in the same manner as in Example1 to obtain a carbon/metal composite. The bending strength of thecomposite was measured. A wear resistance test piece having an 8 mm×8 mmtesting (sliding) surface which was parallel to the surface of the pressduring molding was cut from the composite. A wear resistance test wasperformed by passing a 50 A, 60 Hz current between the testing surfaceof the test piece and a copper plate while rotating the copper plate ata speed of 100 km/hour under a load of 3 kg for 1 hour. Wear resistancewas evaluated by measuring the reduction in the thickness of the testingsurface. The results of measurements are shown in Table 4.

Examples 4 and 5 show the effects of the addition of metal powder to acomposite.

EXAMPLE 4

A molding mixture was prepared using the same coke powder, pitch, andsteel fibers (uncoated) as were used in Example 1. To these materialswere added powder (JIS Grade 1) selected from copper powder (averageparticle diameter=5 μm), nickel powder (average particle diameter=10μm), and cobalt powder (average particle diameter=10 μm). Thecomposition of the resulting molding mixture was 25 parts by weight ofcoke, 10 parts by weight of pitch, 55 parts by weight of steel fibers,and 10 parts by weight of metal powder. The steel fibers constitutedabout 25 volume % of the resulting moldings.

For comparison, molding mixtures were prepared using manganese powderand chromium powder as the metal powder. These powders have a greatertendency to form carbides then does iron. In addition, a molding mixturewas prepared using 65 parts by weight of steel fibers without any metalpowder.

These molding mixtures were molded and baked for carbonization in thesame manner as in Example 1 to obtain carbon/steel composites. Thebending strengths of the resulting composites are shown in Table 5.

As is clear from Table 5, composites containing a metal powder having alow tendency to form carbides had a much higher bending strength thancomposites which did not contain a metal powder. On the other hand, theaddition of manganese or chromium powder resulted in a decrease in thebending strength compared to that of the composite containing no metalpowder.

EXAMPLE 5

Molding mixtures containing copper powder were prepared in the samemanner as in Example 4. The resulting moldings contained 25 volume % ofsteel fibers, and the amount of copper powder which was added was variedbetween 1-10 volume % while maintaining the total content of copperpowder and coke powder at 53 volume %. The composition of each moldingmixture and the bending strength of the resulting composite are shown inTable 6.

As is clear from Table 6, all of the resulting carbon/steel compositeshad a high bending strength. There was a substantial increase in bendingstrength even with the addition of as little as 0.5 volume % of copperpowder.

EXAMPLE 6

In this example, both plated metal fibers and metal powder wereemployed.

Molding mixtures were prepared using the same coke powder, pitch, andcopper-plated or uncoated steel fibers as used in Example 1, and thesame nickel powder as employed in Example 4. Plating was performed byimmersing steel fibers in 20 times their weight of the copper platingsolution employed in Example 1 at 20° C. for 1 hour. The average platingthickness of the copper plating was 0.2 μm.

Molding mixtures having the compositions shown in Table 7 were prepared.300 g of each molding mixture were placed into a steel mold with aninner diameter of 100 mm, and hot pressing was performed under the sameconditions as in Example 3. The resulting moldings were packed in cokepowder and heated in a nitrogen atmosphere from room temperature to1000° C. at a rate of 100° C./hour. After being maintained at 1000° C.for 3 hours, the moldings were cooled. The bending strengths of theresulting carbon/steel composites are also shown in Table 7.

As is clear from Table 7, a composite containing both plated metalfibers and a metal powder has a particularly high bending strength.

Examples 7 and 8 illustrate the use of metal fibers which have beenpreviously alloyed with a metal having a low tendency to form carbides.

EXAMPLE 7

Steel fibers like those used in Example 1 were plated with copper usingthe same plating solution as in Example 1. Plating was performed byimmersing the fibers in 40 times their weight of the plating solution at20° C. for 2 hours. The resulting copper plated fibers had a platingthickness of 0.6 μm.

The copper-plated steel fibers were placed into a porcelain cruciblewhich was then packed with coke powder. The fibers were then heated in anitrogen atmosphere to 1000° C. at a rate of 300° C./hour. After beingmaintained at 1000° C. for 2 hours, the fibers were cooled, and steelfibers alloyed with copper were obtained. These metal fibers were usedto prepare a molding mixture.

The molding mixture comprises of 25 parts by weight of coke powder likethat used in Example 1, 10 parts by weight of pitch like that used inExample 1 (prepared by heating at 450° C.), and 65 parts by weight ofsteel fibers alloyed with copper. The fibers constituted about 30 volume%. For comparison, a molding mixture containing uncoated fibers was alsoprepared.

The molding and baking were performed in the same manner as in Example 1with the exception that 450 g of the molding mixtures were placed in themold and that the molding pressure was 200 kg/cm². A resultingcarbon/steel composite containing alloyed metal fibers had a bendingstrength of 1850 kg/cm², while a composite which contained uncoatedmetal fibers had a bending strength of 1250 kg/cm².

EXAMPLE 8

Steel fibers like those employed in Example 1 were placed into a rotarykiln together with twice their weight of nickel powder having an averageparticle diameter of 20 μm. While being rotated, the fibers and thepowder were heated in a nitrogen atmosphere at a rate of 500° C./hour to900° C., maintained at that temperature for 1 hour, and then cooled. Theremaining nickel powder was separated from the steel fibers, which werenow alloyed with nickel.

Using the steel fibers alloyed with nickel, a molding mixture wasprepared. The preparation of the molding mixture, molding and bakingwere performed in the same manner as in Example 7. The resultingcarbon/steel composite had a bending strength of 1980 kg/cm².

In Examples 9-12, carbon/metal composites were prepared using metalfibers oriented in substantially unidirectional alignment.

EXAMPLE 9

Low carbon steel fibers (SPCC-1B) measuring 0.05 mm×0.05 mm×3 mm longwere immersed at room temperature in 80 times their weight of the copperplating solution used in Example 1, after which they were washed anddried. The resulting copper-plated steel fibers had an average platingthickness of either 0.3 μm or 0.5 μm.

68 parts by weight of the plated steel fibers were mixed with 22 partsby weight of coke powder having an average particle diameter of 15 μmand 10 parts by weight of pitch as binder which was ground to a size of60 mesh or less to obtain a molding mixture. The coke powder wasprepared by the same method as in Example 1. the pitch binder wasobtained by heat treatment of coal tar under a reduced pressure of 100mmHg at 440° C. for 2 hours and had a softening point of 240° C. Thesteel fibers constituted roughly 30 volume % of the molding mixture.

The molding mixture was then placed into a stainless steel mold withinner dimensions of 50 mm×80 mm. Enough molding mixture was used toobtain a molding having a final thickness of 10 mm. The steel fiberswere oriented unidirectionally by applying a magnetic field of 50,000Gauss to the molding mixture while vibrating the mold. The magneticfield was applied such that the line of the magnetic flux is parallel tothe longer side (lengthwise direction) of the mold. Thereafter, moldingby hot pressing was then performed with a hydraulic press having a30-ton capacity. Under a molding pressure of 200 kg/cm², the moldingmixture was heated at 5° C./minute to 550° C., maintained at thattemperature for 1 hour, and then cooled. The resulting molding measured50 mm wide×80 mm long×10 mm thick and contained copper-plated steelfibers which are aligned parallel to the length of the molding.

The molding was placed inside a stainless steel vessel which was packedwith coke powder. The vessel was heated in a nitrogen atmosphere at arate of 10° C./hour to 1000° C., maintained at that temperature for 4hours, and then cooled to obtain a carbon/steel composite.

The bending strength and Charpy impact strength of the composite weremeasured. In addition, a test piece measuring 8 mm wide×8 long×10 mmthick was cut from the composite. One of the 8 mm×8 mm surfaces of thetest piece corresponding to the upper surface of the molding was used asa test surface. A wear test was performed using a sliding speed of 100km/hour (a copper plate rotating at 2000 rpm) under a load of 1.5 kg(2.34 kg/cm²) for 2 hours (200 km of sliding distance). The test piecewas set in the wear tester so that the steel fibers aligned in thecomposite were perpendicular to the direction by sliding. Wearresistance was evaluated by determining the amount of wear (worn volume)of the test place per 100 km of sliding distance. The test results areshown in Table 8.

EXAMPLE 10

A composite containing steel fibers aligned unidirectionally wasprepared in the same manner as in Example 9, with the exception that amixture of uncoated steel fibers and plated steel fibers was employed.The uncoated steel fibers were the same as used in Example 9, while theplated steel fibers were obtained by performing nickel plating of theuncoated steel fibers using the same non-electrolytic nickel platingsolution and the same plating method as in Example 2. The averagethickness of the nickel plating was 1.2 μm. The steel fibers constituted68 parts by weight of the molding mixture, in which 50 parts by weightwere nickel-plated fibers and 18 parts by weight were the uncoatedfibers.

The resulting composite had a bending strength of 2400 kg/cm², a Charpyimpact strength of 18 kgcm/cm², and the amount of wear in the wear testwas 11 mm³ /100 km.

EXAMPLE 11

A composite containing steel fibers aligned unidirectionally wasprepared in the same manner as in Example 9, with the exception that theuncoated steel fibers were employed as the metal fibers and a metalpowder selected from copper, nickel, and cobalt was added to the moldingmixture. The molding mixture contained 19 parts by weight of cokepowder, 9 parts by weight of pitch as a binder, 62 parts by weight ofsteel fibers, and 10 parts by weight of metal powder. The metal fibersconstituted about 30 volume % and the metal powder constituted about 5volume %.

The results of a bending strength test, a Charpy impact test, and a weartest are shown in Table 9.

EXAMPLE 12

A composite containing steel fibers aligned unidirectionally wasprepared in the same manner as in Example 9, except that the uncoatedsteel fibers, copper-plated steel fibers, and/or nickel-plated steelfibers were used as metal fibers. In addition, a metal powder selectedfrom copper, nickel, and cobalt metal powder was added to the moldingmixture. The copper-plated steel fibers were obtained by immersing steelfibers in 20 times their weight of a plating solution and had an averageplating thickness of 0.2 μm. The nickel-plated steel fibers were formedin the same manner as in Example 2 and had an average plating thicknessof 1.2 μm. The compositions of the molding mixtures are shown in Table10. The fibers constituted about 20 volume % and the metal powderconstituted about 5 volume %.

The results of a bending strength test, a Charpy impact test, and a weartest are also shown in Table 10.

EXAMPLE 13

In this example, a carbon/metal composite was manufactured using two ormore types of metal fibers made of different materials or havingdifferent dimensions.

Coke powder having an average particle diameter of 12 μm and pitchbinder which was ground to a size of 60 mesh or less were used as carbonraw materials. The coke powder was obtained by the same method as inExample 1. The pitch binder was obtained by heating coal tar under anabsolute pressure of 60 mmHg at 430° C. for 2 hours. It had a softeningpoint of 270° C. The carbon raw materials were mixed with 1 or 2 typesof metal fibers selected from the following list in the proportionsshown in Table 11 to obtain a molding mixture.

Types of metal fibers

(a) low carbon steel fibers: 0.05 mm×0.05 mm×3 mm long

(b) low carbon steel fibers: 0.1 mm×0.1 mm×6 mm long

(c) low carbon steel fibers: 0.5 mm×0.5 mm×25 mm long

(d) copper fibers: 0.05 mm×0.05 mm×3 mm long

Each molding mixture was placed into a mold having inner dimensions of100 mm×200 mm. Under a pressure of 200 kg/cm², the temperature of themolding mixture was raised at a rate of 5° C./min to 500° C. andmaintained at that temperature for 1 hour. After cooling, moldingsmeasuring 100 mm×200 mm ×10 mm thick was removed from the mold. Themoldings were placed in a coke powder in a nitrogen atmosphere, heatedat a rate of 12° C./hour to 1000° C., maintained at that temperature for4 hours, and then cooled to obtain carbon/steel composites.

Measurements of the bending strength, the charpy impact strength, thewear resistance, and the electric resistance are also shown in Table 11.Wear resistance was measured under the same conditions as in Example 9.As shown in Table 11, a composite containing a mixture of two differenttypes of metal fibers has excellent wear resistance and impactresistance, and a low electric resistance.

EXAMPLE 14

In this example, a carbon/metal composite was manufactured using twotypes of metal fibers made of different materials which were disposed indifferent portions of the composite along its thickness.

Coke powder having an average particle diameter of 12 μm and a binderpitch having a softening point of 250° C. and ground to a size of 60mesh or less were used as carbon raw materials. The coke powder was thesame as that used in Example 13. The pitch was obtained by heattreatment of coal tar under absolute pressure of 100 mmHg at 420° C. for6 hours. Two kinds of metal fibers used were: (a) low carbon steelfibers and (b) copper fibers both measuring 0.05 mm×0.05 mm×3 mm long.

Fibers (a) and (b) were separately mixed with the carbon raw materialsin the proportions shown in Table 12 to obtain four types of moldingmixtures A-D. Two of the four types of mixtures were then stacked oneabove the other in a mold having internal dimensions of 100 mm×200 mm.Each mixture was used in an amount sufficient to give a thickness of 5mm after molding so that a molding having a thickness of about 10 mm andcontaining different types of fibers between the upper half and thelower half was formed.

Molding and baking were performed in the same manner as in Example 13except that the hot pressing temperature was 550° C., and carbon/steelcomposites were obtained.

The types of molding mixtures used in each composite along with thebending strength, the wear resistance, and the electric resistance ofthe resulting composites are shown in Table 13 below. Wear resistancewas measured in the same manner as in Example 9 except that the testsurface of the test piece was the upper surface of the molding which isperpendicular to the pressing direction during molding. It can be seenthat the resulting composites all had excellent wear resistance and alow electric resistance.

EXAMPLE 15

In this example, a carbon/metal composite was manufactured in which thecontent of steel fibers was varied along the thickness of the composite.

The same coke powder as used in Example 13 (average particle diameter=12μm), the same pitch binder used in Example 14, and low carbon steelfibers measuring 0.05 mm×0.05 mm×3 mm long were mixed in the proportionsshown in Table 14 below to obtain four different molding mixtures A-Dhaving different contents of steel fibers. Two of the four types ofmixtures were stacked one above the other in a mold with internaldimensions of 100 mm×200 mm each to a depth sufficient to give athickness of 5 mm after molding. Thus, the content of metal fibers wasdifferent for the upper and lower portions of the mold.

The molding mixtures were hot pressed and baked in the same manner as inExample 14 to obtain carbon/steel composites having a thickness of about10 mm.

The types of molding mixtures employed in each composite and the bendingstrength, wear resistance, and Charpy impact strength are shown in Table15. The wear resistance was measured in the same manner as in Example14. From Table 15, it can be seen that the composites according to thepresent invention had excellent wear resistance and impact strength.

EXAMPLE 16

In this example, carbon/metal composites were manufactured in which thedimensions of the metal fibers varied along the thickness of thecomposite.

The same coke powder and pitch binder as in Example 9 were used ascarbon raw materials. These were mixed with one of the following threetypes of steel fibers having different dimensions in the proportionsgiven in Table 16 below to obtain six different types of moldingmixtures A-F. The dimensions of the three different types of fibers wereas follows:

(a) 0.05 mm×0.05 mm×3 mm long (eq.D=0.056 mm)

(b) 0.1 mm×0.1 mm×6 mm long (eq.D=0.113 mm)

(c) 0.5 mm×0.5 mm×25 mm long (eq.D=0.564 mm) (eq.D: equivalent diameter)

Two of the six types of mixtures were stacked one above the other in amold having internal dimensions of 100 mm×200 mm each to a depthsufficient to give a thickness of 5 mm after molding. A mixturecontaining longer and thicker fibers was placed in the lower half whilea mixture containing shorter and thinner fibers was placed in the upperhalf of the mold.

The molding mixtures were molded by hot pressing and baked under thesame conditions as in Example 14 to obtain carbon/steel compositeshaving a thickness of about 10 mm.

The types of molding mixtures employed in each composite along with theelectric resistance, bending strength, wear resistance, and Charpyimpact strength of the composites are shown in Table 17 below. The wearresistance was measured in the same manner as in Example 9. As is clearfrom Table 17, the composites had excellent wear resistance and impactstrength.

While the invention has been described with reference to the foregoingembodiments, various changes and modifications can be made thereto whichfall within the scope of the appended claims.

                                      TABLE 4                                     __________________________________________________________________________            Parts by weight    Properties                                                 Uncoated                                                                           Cu-plated     Bending   Electric                                                                           Reduction in                                steel                                                                              steel Coke    strength                                                                           Density                                                                            resistance                                                                         thickness after                     Run No. fiber                                                                              fiber powder                                                                             Pitch                                                                            (kg/cm.sup.2)                                                                      (g/cm.sup.2)                                                                       (μΩcm)                                                                    wear test (mm)                      __________________________________________________________________________    1 This   42  100   48   22 1850 3.6  110  0.20                                  invention                                                                   2 Compara-                                                                            142  --    48   22 1210 3.7  120  0.18                                  tive                                                                        3 This  --   142   48   22 1650 3.8   95  0.30                                  invention                                                                   __________________________________________________________________________

                  TABLE 5                                                         ______________________________________                                                              Bending                                                                       strength                                                Run No.   Metal powder                                                                              (kg/cm.sup.2)                                                                          Remarks                                        ______________________________________                                        This    1     Copper      1710   Metal powder with                            Invention                                                                             2     Nickel      1940   tendency to form                                     3     Cobalt      1800   carbides equal to                                                             or lower than iron                           Compara-                                                                              4     Steel fibers                                                                              1250   No metal powder                              tive    5     Manganese    440   Metal powder with                                    6     Chromium    1170   tendency to form                                                              carbides greater                                                              than iron                                    ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                               Parts by weight                                                               (Volume percent in parentheses)                                                                    Bending                                                    Steel                          strength                              Run No.  fiber  Cu powder Coke powder                                                                            Pitch                                                                              (kg/cm.sup.2)                         ______________________________________                                        This   1     25     10      43       22   1640                                Invention    (120)  (56)    (43)     (22)                                            2     25     5       48       22   1710                                             (120)  (28)    (48)     (22)                                            3     25     2       51       22   1670                                             (120)    (11.2)                                                                              (51)     (22)                                            4     25     1       52       22   1720                                             (120)    (5.6) (52)     (22)                                            5     25       0.5     52.5   22   1510                                             (120)    (2.8)   (52.5) (22)                                     ______________________________________                                    

                                      TABLE 7                                     __________________________________________________________________________              Parts by weight           Bending                                             Cu-plated                                                                           Uncoated    Coke    strength                                  Run No.   steel fiber                                                                         steel fiber                                                                         Ni powder                                                                           powder                                                                             Pitch                                                                            (kg/cm.sup.2)                             __________________________________________________________________________    This invention                                                                        1 120   --    10    48   22 2010                                              2 --    132   10    48   22 1750                                              3  32   100   10    48   22 1980                                      Comparative                                                                           4 --    142   --    48   22 1210                                      This invention                                                                        5 142   --    --    48   22 1650                                      __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________                      Average plating                                                                       Bending                                                                            Charpy impact                                                                         Amount of                                        Unidirectional                                                                        thickness                                                                             strength                                                                           strength                                                                              wear                                   Run No.   orientation                                                                           (μm) (kg/cm.sup.2)                                                                      (kgcm/cm.sup.2)                                                                       (mm.sup.3 /100 km)                     __________________________________________________________________________    This invention                                                                        1 Oriented                                                                              0.3     2050 15      10                                             2 "       0.5     2250 17      12                                     Comparative                                                                           1 None    --      1050  5      11                                             2 Oriented                                                                              --      1450 12      12                                     __________________________________________________________________________

                                      TABLE 9                                     __________________________________________________________________________                              Bending                                                                            Charpy impact                                                                         Amount of                                        Unidirectional                                                                        Metal powder                                                                          strength                                                                           strength                                                                              wear                                   Run No.   orientation                                                                           added   (kg/cm.sup.2)                                                                      (kgcm/cm.sup.2)                                                                       (mm.sup.3 /100 km)                     __________________________________________________________________________    This invention                                                                        1 Oriented                                                                              Copper  2300 18       9                                             2 "       Nickel  2550 20      13                                             3 "       Cobalt  2350 17      12                                     Comparative                                                                           1 None    --      1050  5      11                                             2 Oriented                                                                              --      1450 12      12                                     __________________________________________________________________________

                                      TABLE 10                                    __________________________________________________________________________              Volume percent (parts by weight in parentheses)                                                               Bending                                                                            Amount of                      Unidirectional                                                                          Steel fiber     Metal   Coke    strength                                                                           wear                           orientation                                                                             Cu-plated                                                                           Ni-plated                                                                          Uncoated                                                                           powder  powder                                                                             Pitch                                                                            (kg/cm.sup.2)                                                                      (mm.sup.3 /100                 __________________________________________________________________________                                                   km)                            1 Oriented                                                                              20    --   --   Ni: 5   52   23 2700 9.0                                      (100)           (28)    (52) (23)                                   2 "       10    --   10   Ni: 5   52   23 2300 8.5                                      (50)       (50) (28)    (52) (23)                                   3 "       10    --   10   Cu: 5   52   23 1850 5.2                                      (50)       (50) (29)    (52) (23)                                   4 "       20    --   --   Co: 5   52   23 1900 8.1                                      (100)           (28)    (52) (23)                                   5 "       --    20   --   Cu: 5   52   23 2600 7.3                                            (100)     (29)    (52) (23)                                   6 "       10    --   10   Cu: 2.5                                                                           Ni: 2.5                                                                           52   23  2100                                                                              8.1                                      (50)       (50) (14)                                                                              (14)                                                                              (52) (23)                                   7 "       10    10   --   Cu: 5   52   23 1950 6.7                                      (50)  (50)      (29)    (52) (23)                                   1 None    --    --   20   --      57   23 750  7.2                                                 (100)        (52) (23)                                   2 Oriented                                                                              --    --   20   --      57   23 1350 8.4                                                 (100)        (52) (23)                                   __________________________________________________________________________

                                      TABLE 11                                    __________________________________________________________________________                                                 Charpy                                   Volume percent          Bending                                                                            Amount of                                                                             impact Electric                          Fiber                                                                             Fiber                                                                             Fiber                                                                             Fiber                                                                             Coke    strength                                                                           wear    strength                                                                             resistance                Run No. (a) (b) (c) (d) powder                                                                             Pitch                                                                            (kg/cm.sup.2)                                                                      (mm.sup.3 /100 km)                                                                    (kgcm/cm.sup.2)                                                                      (μΩcm)           __________________________________________________________________________    This  1 20  10          49   21 1140 12      6.7    100                       invention                                                                           2 20      10      49   21 1070 15      14.0   102                             3 20          10  49   21 1120 16      4.6    48                              4     20  10      49   21 1130 18      16.2   96                        Compara-                                                                            5 30              49   21 1200 11      5.0    105                       tive  6     30          49   21 1090 14      7.2    98                              7         30      49   21  820 27      19.0   96                              8             30  49   21  980 44      4.3    30                        __________________________________________________________________________

                  TABLE 12                                                        ______________________________________                                        Volume percent                                                                Blend Steel fiber Copper fiber                                                                             Coke powder                                                                              Pitch                                 ______________________________________                                        A      0          30         48         22                                    B      0          40         38         22                                    C     30           0         48         22                                    D     40           0         38         22                                    ______________________________________                                    

                  TABLE 13                                                        ______________________________________                                                                  Amount of                                                  Combination                                                                             Bending  wear      Electric                                         of blends strength (mm.sup.3 /                                                                             resistance                                Run No.  Upper   Lower   (kg/cm.sup.2)                                                                        100 km) (μΩcm)                       ______________________________________                                        This   1     C       A     1090   13      48                                  invention                                                                            2     D       A     1150   12      32                                         3     C       B     1230   12      45                                         4     D       B     1290   13      31                                  Com-   5     A       A      940   35      30                                  parative                                                                             6     B       B     1020   48      20                                         7     C       C     1220   12      110                                        8     D       D     1450   13      90                                  ______________________________________                                    

                  TABLE 14                                                        ______________________________________                                                 Volume percent                                                       Blend    Coke powder     Fiber   Pitch                                        ______________________________________                                        A        48              30      22                                           B        38              40      22                                           C        28              50      22                                           D        18              60      22                                           ______________________________________                                    

                  TABLE 15                                                        ______________________________________                                                                            Charpy                                                              Amount of impact                                           Combination                                                                             Bending  wear      strength                                         of blends strength (mm.sup.3 /                                                                             (kgcm/                                    Run No.  Upper   Lower   (kg/cm.sup.2)                                                                        100 km) cm.sup.2)                             ______________________________________                                        This   1     A       C     1240   13      10.7                                invention                                                                            2     A       D     1480   12      12.7                                       3     B       C     1520   12      11.2                                Com-   4     A       A     1090   12      7.3                                 parative                                                                             5     B       B     1360   13      8.2                                        6     C       C     1450   18      12.4                                       7     D       D     1540   29      15.2                                ______________________________________                                    

                  TABLE 16                                                        ______________________________________                                        Fiber size                                                                    (Equivalent diameter ×                                                                    Volume percent                                              Blend length) (mm)    Fiber   Coke powder                                                                            Pitch                                  ______________________________________                                        A     0.056 × 3 30      50       20                                     B     0.113 × 6                                                         C      0.564 × 30                                                       D     0.056 × 3 40      40       20                                     E     0.113 × 6                                                         F      0.564 × 30                                                       ______________________________________                                    

                                      TABLE 17                                    __________________________________________________________________________             Combination                                                                           Amount of                                                                             Charpy impact                                                                         Bending                                                                            Electric                                         of blends                                                                             wear    strength                                                                              strength                                                                           resistance                              Run No.  Upper                                                                             Lower                                                                             (mm.sup.3 /100 km)                                                                    (kgcm/cm.sup.2)                                                                       (kg/cm.sup.2)                                                                      (μΩcm)                         __________________________________________________________________________    This   1 A   B   11      7.2     1100 80                                      invention                                                                            2 A   C   11      13.1    1003 83                                             3 D   E   13      10.6    1380 68                                             4 D   F   12      19.2    1220 70                                      Comparative                                                                          5 A   A   11      5.1     1120 82                                             6 B   B   18      7.8     1010 80                                             7 C   C   86      20.2     850 105                                            8 D   D   13      9.2     1430 69                                             7 E   E   22      12.0    1260 73                                             8 F   F   140     28.0     930 70                                      __________________________________________________________________________

What is claimed is:
 1. A carbon/metal composite comprising a carbonmatrix and metal fibers distributed in the carbon matrix, the surfacesof at least a portion of the metal fibers being coated or alloyed with ametal having a tendency to form carbides equal to or less than the metalfibers and which suppresses formation of carbides in the metal fibers.2. A carbon/metal composite as claimed in claim 1, wherein the metalfibers are those of rod-shaped, needle-shaped, wedge-shaped,wave-shaped, net-shaped, or a mixture of these shapes.
 3. Thecarbon/metal composite of claim 1, wherein the composite has a bendingstrength of at least 1420 kg/cm².
 4. The carbon/metal composite of claim1, wherein the composite has an impact strength of at least 15 kgcm/cm².5. The carbon/metal composite of claim 1, wherein the composite has awear resistance of less than 13 mm³ per 100 km of sliding distancewherein a test surface of the composite is rotated at 100 km/houragainst a copper plate rotating at 2000 rpm under a load of 1.5 kg. 6.The carbon/metal composite of claim 1, wherein the composite has anelectrical resistance of less than 110 μΩcm.
 7. The carbon/metalcomposite of claim 1, wherein the fibers have a diameter of no greaterthan 0.5 mm and a length of at least 1 mm.
 8. The carbon/metal compositeof claim 1, wherein the fibers have a diameter of no greater than 0.3 mmand a length of at least 3 mm.
 9. The carbon/metal composite of claim 1,wherein the fibers have an aspect ratio of at least
 10. 10. Thecarbon/metal composite of claim 1, wherein the fibers are present in anamount of at least 10% by volume of the composite.
 11. The carbon/metalcomposite of claim 1, wherein the fibers are present in an amount of 10to 40% by volume of the composite.
 12. The carbon/metal composite ofclaim 1, wherein the material coated on the fibers has a thickness of atleast 0.1 μm.
 13. The carbon/metal composite of claim 1, wherein atleast 50% by weight of the fibers are coated.
 14. A carbon/metalcomposite comprising a carbon matrix and metal fibers distributed in thecarbon matrix, the surfaces of at least a portion of the metal fibersbeing coated or alloyed with a metal having a tendency to form carbidesequal to or less than the metal fibers and which suppresses formation ofcarbides in the metal fibers, said metal fibers being steel fibers andsaid barrier metal being one or more metals selected from the groupconsisting of copper, nickel, cobalt, aluminum, and silicon.
 15. Acarbon/metal composite as claimed in claim 14, wherein a steel fibersare low carbon steel fibers.
 16. A carbon/metal composite comprising acarbon matrix and metal fibers distributed in the carbon matrix, thesurfaces of at least a portion of the metal fibers being coated oralloyed with a material having a tendency to form carbides equal to orless than the metal fibers and which suppresses formation of carbides inthe metal fibers, the surface of at least a portion of the metal fibersbeing coated with said material.
 17. A carbon/metal composite as claimedin claim 1, wherein the surface of at least a portion of the metalfibers is alloyed with said metal.
 18. A carbon/metal composite asclaimed in claim 1, wherein the metal fibers are distributed in thecomposite so as to be substantially undirectional with respect to eachother.